This invention relates to frequency conversion of laser light.
In some situations, it is desired to convert the wavelength of a laser beam to one or more higher wavelength values (i.e., introduce a xe2x80x9cred shiftxe2x80x9d), where a coherent light source is needed in a spectral region where a primary source is not available. A red shift may be introduced to minimize optical absorption that would otherwise seriously degrade the intensity of an incident light beam at the lower wavelength, for example, in a nonlinear optical semiconductor crystal. In another situation, introducing a red shift may allow the modified laser beam to hit certain target molecular absorption resonances that are to be studied.
Red shift of laser radiation can be achieved using stimulated Raman scattering (SRS) in gases, liquids or solids, as described by W. Koechner in Solid State Laser Engineering, Springer Verlag, Berlin, N.Y., 1988, pp. 526-535. In an SRS system, an incident laser beam having a selected frequency xcexd0 interacts with a selected material and a light beam having a Stokes-shifted frequency xcexd0xe2x88x92xcex94xcexd issues from the material, where xcex94xcexd is a frequency difference that is characteristic of the material. A selected wavelength, xcex=1.064 xcexcm, of an Nd:YAG laser can be converted, using SRS in a medium such as H2, which provides a vibrational wavenumber (difference) xcex94xcexdxe2x80x2=4155 cmxe2x88x921, to a resulting wavelength xcex=1.91 xcexcm radiation, for example.
Red shifting of laser radiation can also be implemented using an optical parametric oscillator (OPO) in which the xe2x80x9cpumpxe2x80x9d laser photon decays into two smaller energy photons, referred to as the xe2x80x9csignalxe2x80x9d and the xe2x80x9cidler.xe2x80x9d An OPO approach creates coherent light that is red shifted with respect to the original xe2x80x9cpumpxe2x80x9d laser photon.
Using these traditional methods for red shifting laser radiation, it is difficult to achieve high conversion efficiency (≈100 percent) because of the presence of a quantum defect; the red shifted signal photon carries away less energy than was present in the original pump photon. This defect may be especially critical when the frequency of the red shifted signal photon is much less than the frequency of the pump photon.
What is needed is a red shift process that provides more than one red shifted photon, all with approximately the same photon energy. Preferably, this approach should be capable of providing two or three signal photons having substantially the same frequency, where the sum of the energies associated with the photons produced is approximately equal to the pump photon energy.
These needs are met by the invention, which provides a cascaded frequency down-conversion in which each pump photon produces more than one red-shifted photon. In one embodiment, passage of a pump photon (3xcfx89) through a selected first nonlinear crystal produces a first parametric conversion (3xcfx892xcfx89+xcfx89); and passage of the resulting 2xcfx89+xcfx89 photons through a selected second nonlinear crystal produces a second parametric conversion (2xcfx89+xcfx89xe2x86x92xcfx89+xcfx89+). The first and second nonlinear crystals are aligned within a cavity resonating at frequency 2xcfx89, which is trapped within the cavity until its conversion to xcfx89+xcfx89 photons that may exit from the cavity. In another embodiment, the first and second parametric conversion processes are (4xcfx89xe2x86x922xcfx89+2xcfx89) and (2xcfx89xe2x86x92xcfx89+xcfx89).